Example 3.2.4.6. Let $X$ be a simplicial set, and let $\emptyset $ denote the empty simplicial set. Then there is a unique morphism of simplicial sets $\emptyset \hookrightarrow X$, which is nullhomotopic if and only if $X$ is nonempty (note that, by the convention of Definition 3.2.4.5, the identity map $\emptyset \rightarrow \emptyset $ is not considered to be nullhomotopic).
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$